This invention relates to coolant pumps for automotive internal-combustion engines. The invention is aimed at providing a coolant pump which delivers flow characteristics in accordance with engine demand.
Pumps for internal-combustion engine cooling systems have traditionally been belt-driven, at a fixed ratio, directly from the engine.
The coolant flow rate and pressure head required to effectively control the engine temperature are not, however, optimal when driven proportionally to the engine""s rotational speed. The coolant system has to cope with the fully-laden vehicle struggling up-hill on a hot day, and the same system has to make sure the heater warms up rapidly in very cold conditions. Also, for efficiency, the energy consumed by the coolant pump ideally should at all times be only the minimum needed to just achieve the optimum temperature in the coolant. Whatever coolant circulation system is used, it must of course cater for the extremes; in the case of the traditional belt-driven coolant pumps, the need to cater for the extremes so compromises the efficiency of normal running that traditional coolant pumps are inherently non-optimal for most of their operating conditions.
The optimum coolant temperature is dictated by considerations of engine performance, fuel efficiency, exhaust emissions, etc. The coolant circulation system must provide a volumetric flow rate, and a pressure head, such that the coolant is cooled down (or warmed up) to the correct temperature under the extreme conditions. The invention is aimed at making it possible still to accommodate the extremes, and yet to improve the efficiency of the coolant circulation system during normal running, so that the system consumes only a minimum of energy during normal running.
When the coolant pump provides excessive flow and head, the engine wastes power and the overall engine efficiency is reduced.
When the coolant pump provides insufficient coolant flow and head, the engine runs too hot, thereby reducing engine performance, and perhaps damaging the engine.
Engine designers have not, in general, switched to driving coolant pumps by means of electric motors. This fact should be viewed in light of the fact that it is very common for a designer to specify that the engine""s cooling fan to be driven by an electric motor. There, the motor runs at constant speed, and is controlled simply by being switched on/off: the need for switching is signalled by a simple electrical thermostat. That is a simple enough duty requirement for an electric motor to be subjected to.
It is recognised, however, that a simple on/off control would be far too crude for controlling the flow of coolant. Even under the minimum coolant flow conditions, the coolant must still be pumped and circulated quite vigorously.
It might be considered that, if an electrically-driven coolant pump were to be provided, it would be possible to control the coolant flow by controlling the rotational speed of the electric motor. Theoretically, this could be done by varying the electric current supplied to the motor that drives the coolant pump. However, such control of the motor speed by control of the motor current has not found favour with engine designers.
Thus, in considering the use of an electric motor to drive the coolant pump, it is apparent, first, that simple thermostatic on/off switching of a pump motor is out of the question, and second, trying to control motor-speed by controlling the current supplied to the electric motor has not found favour. And, even as a last resort, the notion of controlling coolant-flow by means of coupling the pump to a fixed speed motor by means of a mechanical variable speed drive, must be contra-indicated out as being far too elaborate; also, as mentioned, it is important that the pump, as well as the motor, should run at constant speed.
The invention is aimed at making it possible to vary the coolant flow to suit many different conditions, in a way which allows the pump (and hence the motor) to run at constant speed.
The design configurations as will be described herein employ variable pitch guide vanes to affect the velocity, flow rate, pressure head, etc. of the coolant. The guide vanes are located adjacent to the impeller of the coolant pump, in the flow of coolant as it passes through the pump. The vanes are operated in response to a temperature signal corresponding to the actual cooling demand of the engine. The guide vanes serve to boost or to reduce the flow of coolant through the impeller, the change between boost and reduce being effected as a consequence of a change in the positional orientation of the vane in relation to the impeller of the pump.
The heat rejection demand is made dependent upon the temperature of the system, not engine speed. The system temperature might, for example, be taken as the temperature of the cooling fluid, or the temperature of a particular location on a machine, such as near the exhaust valves on the cylinder heads of an internal combustion engine. The system temperature may be transduced into a mechanical displacement which adjusts the pitch of a set of the guide vanes, which are preferably located just upstream of the impeller. When the system temperature is high, the thermostatic transducer adjusts the vanes such that the impeller pump provides a high coolant flow rate; when the system temperature is low the vanes are adjusted to provide a lower coolant flow rate.
It should be noted that, in an internal combustion engine, it is required that the coolant flow be maintained at all times during operation of the engine. The minimum flow demand is still a substantial flow. The engine would overheat in a few seconds if flow were actually to stop. Thus, it will be understood that the flow rate being controlled is just the upper fraction of the maximum flow ratexe2x80x94an area of flow in which it is notoriously difficult for a designer to achieve a desired degree of linearity of control. It is recognised that controlling just the upper fraction of the flow rate is not only easy with the variable pitch vanes, but, when the vanes are moved, the change in flow rate is not too far from being more or less linearly proportional to the movement of the vane. This means that simple automotive wax-type thermostats can be used directly, since they too have a more or less linear temperature/movement characteristic.
The use of variable pitch guide vanes combined with a modern high-speed impeller produces increased hydrodynamic flow efficiency over a wide range of flow rates, and provides capability to reduce the flow rate when the demand decreases. In contrast to a conventional direct drive impeller pump which frequently provides excessive coolant flow and uses excessive power, the temperature-responsive variable vane system as described herein, can provide precisely the correct amount of coolant flow to maintain optimum system operating temperatures, while consuming less power.
This pump""s variable hydrodynamic flow/pressure capability, even though driven at a reasonably constant speed, provides thermal controllability while eliminating the need for a variable or multiple speed electrical motor. Increased hydrodynamic flow efficiency combined with the use of small high-speed motors can result in the overall pump package being small, lightweight, efficient, and easy to integrate within a given cooling system""s special constraints.
The thermostatic signal can be transduced directly into a mechanical displacement of the guide vanes, for simple systems. For more sophisticated systems, a thermal signal can be processed by the engine management system which then controls an electrically-activated displacement mechanism to adjust the guide vanes.